Typically, a demapper plays the following roles: after a channel compensation is executed for a received data, the demapper generates an input of a channel decoder with an appropriate size about a number of bits of a soft decision based on modulation after various demapping processes.
After the demapping herein, the reason why the output of the demapper is set as the rather than a hard decision is that the soft decision input which is applied to the channel decoder has a performance gain 2.2 dB theoretically in an Additive White Gaussian Noise (AWGN) channel with respect to the hard decision input.
However, in any case increasing the number of bits of the soft decision in the demapper, the gain will not continue getting bigger. The outputted number of bits of the demapper is determined by a memory size of the deinterleaver behind the demapper in a typical communication system. Therefore, the design for the demapper focuses on the following aspects: to minimize performance degradation and to less dispatch the number of output bits.
In order to achieve foregoing objectives, the conventional demapper has the following configuration: computing an appropriate location of the channel-compensated data in advance, thereby outputting in accordance with a degree of the number of bits of the soft decision from the corresponding location. The channel-compensated data is utilized as the input of the demapper according to a reference gain value of an automatic gain adjusting device in front of the demapper
This is a method of determining an average level of the channel-compensated data by using the reference gain value of the automatic gain adjusting device. In the case that channel conditions are at a normal level, a good performance can be expected.
However, in the case that the channel conditions change to an unexpected state, or interfering signals such as an interchannel interference or a co-channel interference and so on exist, an input level of the demapper may substantially change. Therefore, if the method of only using a reference level of the automatic gain adjusting device to determine the output of demapper at a predetermined position is utilized in this case, an abnormal output of the demapper is outputted.
For example, when the input of the demapper is larger than normality, the majority of the output of the demapper is dispatched in a maximum output. Conversely, when the input of the demapper is smaller than the normality, the majority of the output of the demapper is transmitted in zero. In this case, the performance degradation of a whole receiver is unavoidable.
In order to reduce the performance degradation in the aforesaid case, the number of output bits of the demapper should be further enlarged to a sufficient number of bits for ensuring a response to the unexpected channel conditions. However, as mentioned above, because the deinterleaver is positioned behind the demapper in the typical communication system, the memory of the deinterleaver will get bigger based on the degree. Thus, it is not a good solution.